1. The Field of the Invention
The subject matter of the invention includes a solarization-resistant glass composition having a UV-cutoff with a predetermined transmittance gradient and a radiating device for a weathering apparatus containing a glass with the aforesaid glass composition.
2. The Description of the Related Art
Man-made weathering apparatus simulates intense solar radiation and if necessary additional climatic conditions, such as high temperatures, high or low humidity, corrosive atmospheres (ocean climate) and so forth.
A weathering apparatus is used to perform tests, especially to determine the resistance of lacquers, polymers, pigments, paints, light stabilizers, plastics and the like to aging, in order to ascertain their service life during exposure to UV and visible radiation.
The radiating device in the weathering apparatus, which should simulate solar radiation, comprises a lamp with a radiation source or light source that produces radiation or light, e.g. a metal vapor lamp or a high-power Xenon lamp. These lamps can have a respective power of up to 60 kW without more.
The radiation source is arranged within an inner glass tube in the radiating device. An additional tube, the outer tube, is arranged surrounding the inner glass tube and spaced from it. A cooling fluid, usually water, flows continuously through the hollow chamber between the inner and the outer tubes for cooling the apparatus in order to maintain a constant temperature in the weathering apparatus.
The inner glass tube and/or the outer glass tube should have a UV-cutoff in a wavelength range of about 280 nm to about 330 nm, which has a predetermined transmittance gradient. The term “transmittance gradient” means the slope of the dependence of the transmission on wavelength.
According to the state of the art frequently glass is used in which a classic UV blocking by means of TiO2 or CeO2 takes place. Generally the transmittance gradient of the UV-cutoff is too steep in the case of this sort of classical UV blocking (using TiO2) and/or the glass has a tendency toward solarization, which is too great (using CeO2). It is also possible to influence the UV-cutoff by doping the glass with Fe2O3. A carbon arc lamp for a weathering apparatus is known e.g. from JP 03-218940, which comprises a glass doped with Fe2O3, TiO2 and Sb2O3. This glass contains 5 to 80 ppm of Fe2O3, 50 to 1000 ppm of TiO2 and 0.02 to 1% of Sb2O3 in a borosilicate glass with a composition comprising 65-80%, SiO2; 1-10%, Al2O3; 4-12%, ΣLi2O+Na2O+K2O; 8-20%, B2O3 and 0-10%, ΣCaO+MgO+BeO+ZnO.
The total transmission of a Fe2O3-containing glass is generally reduced as the glass ages under intense solarization, i.e. by action of UV light. That is disadvantageous because the radiating device or the weathering apparatus ages so that uniform irradiation of the test samples is not guaranteed. For that reason the meaningfulness and the reproducibility of the test results obtained with that sort of weathering apparatus are not guaranteed.